148 THE CIRCULATION. 



of a healthy pulse in which the tone of the vessel is better 

 than in the last instance, and the down-stroke is therefore 

 less interrupted. 



Sometimes the up-stroke has a double apex, as in fig. 

 46. This will be explained hereafter. 



Before proceeding to consider the formation of the pulse, 

 as shown by these tracings, it is necessary to consider what 

 are the elements combined to produce it. 



The heart at regular intervals discharges a certain 

 quantity of blood into the arteries and their branches, 

 already filled, though not distended to the utmost, with 

 fluid. This fresh quantity of blood obtains entrance by 

 the yielding of the artery's elastic walls, and, on the 

 cessation of the propelling force, and when these walls 

 recoil, the blood is prevented from returning into the 

 ventricle whence it is issued, by the shutting of the semi- 

 lunar valves in the manner before described (p. 1 17). The 

 pressure, therefore, which is exercised on the blood by the 

 contracting arterial walls, will cause it to travel in a direc- 

 tion away from the heart, or, in other words, towards the 

 capillaries and veins. 



It was formerly supposed that the pulse was caused not 

 by the direct action of the ventricle, but by the propaga- 

 tion of a wave in consequence of the elastic recoil of the 

 large arteries, after their distension ; and successive acts of 

 dilatation and recoil, extending along the arteries in the 

 direction of the circulation, were supposed to account for 

 the later appearance of the pulse in the vessels most 

 distant from the heart. The fact, however, that the pulse 

 is perceptible in every part of the arterial system previous 

 to the occurrence of the second sound of the heart, that is, 

 previous to the closure of the aortic valves, is a fatal 

 objection to this theory. For, if the pulse were the effect 

 of a wave propagated by the alternate dilatation and con- 

 traction of successive portions of the arterial tube, it ought, 

 in all the arteries except those nearest to the heart, to 



